The best deal is probably to get Trilogy, which is the original Guild Wars, Factions and Nightfall all bundled together. Each game can be played on it's own, but they are designed to link together allowing you to share characters, skills and items across all three games.

If you have a choice I recommend starting with Guild Wars (also called Prophecies). Being the first in the series Guild Wars introduces you to the game and starts off the whole storyline. Everything will make more sense if you start at the beginning.

Standalone expansion is when the expansion content does not interact with the vanilla content, like the Gothic expansions.

It's a campaign.

It's not a separate game, because it's just part of the whole game.
And it's not an expansion, because you don't need a base content to play it.

So they decided to call it a campaigns.
Campaigns have starting areas and can be played without having any other content of the game, but they are still way better and more enjoyable if you haver all of the content unlocked.

If I have Guild Wars in my Steam account, which is that? Is that just the original or all combined?

In order to find out, you should look at your License for the game. It will tell you whether the copy of Guild Wars you have is only the first Guild Wars (also called Guild Wars: Prophecies), or the Guild Wars Trilogy.